1. The Invention
This invention relates to transistors and particularly to a new type of structure for a microwave field effect transistor (FET). This invention was made in the performance of work under a contract with the U.S. Department of the Navy.
2. Description of the Prior Art
Field effect transistors of the MESFET (metalsemiconductor-FET) type generally comprise a body of semiconductive material, the upper layer at least of which is epitaxially grown and which has formed on the upper surface thereof a plurality of conductive electrodes, generally source, gate, and drain electrodes. The source and drain electrodes are generally ohmic contacts to the surface of the epitaxial layer while the gate electrode is generally formed to provide a rectifying or Shottky barrier contact so as to create a depletion region between the source and drain, whereby changing the bias on the gate can control the carriers in this region and thereby control the source-drain current and hence the gain of the FET.
Contacts to the surface electrodes (source, gate, and drain) are usually made by the use of wire leads extending from electrodes on the chip to contact lands spaced from the chip and mounted on an adjacent circuit. This method of contacting the FET has several disadvantages.
For one, the wire leads from electrodes to lands have significant inductance and resistance at microwave frequencies. As is known, the provision of such series inductance and resistance in association with active microwave elements raises the noise figure of the active element and limits the maximum frequency of operation, i.e., the maximum frequency at which usable gain can be obtained from the active element.
Another disadvantage of the prior art arrangement is that heat generated adjacent the source electrode was not able to escape readily from the device due to the fact that the thin wire leads do not conduct significant heat and the actual semiconductor body of the device itself is not a good thermal conductor. Thus the thermal resistance of the device, which is defined as the amount of temperature rise of the device per unit of power dissipated in the device, was high, being generally about 50.degree. centigrade per watt. Such a high thermal resistance severely limits the RF power capability of the device since the amount of power which can be handled by the device is limited by the maximum safe operating temperature of the device. As is known, a reduction of the thermal resistance of the device would increase the amount of power which could be handled by the device while still operating under its safe maximum temperature.
Another disadvantage of the need for wire leads on top of the device is the necessity of laying out such wire leads and the respective electrodes to which they are attached such that the leads do not contact each other. If it were possible to eliminate one or more of such leads, extra freedom would be provided in laying out the remaining wire leads and the respective electrodes such that shorter leads could be used, thereby to lower lead inductance and resistance; also the geometry or configurations of the top electrodes could be adjusted or changed in order to optimize such geometry and concomitant device performance.